It is known to mold hard (i.e., permanent) magnets as well as soft magnetic cores for electromagnetic devices (e.g., transformers, inductors, motors, generators, relays, etc.) from ferromagnetic particles dispersed throughout a polymer matrix. Soft magnetic cores are molded from ferromagnetic particles such as iron, and certain silicon, aluminum, nickel and cobalt alloys thereof (hereafter generally referred to as iron) dispersed throughout a thermoset or thermoplastic polymer matrix. Such cores serve to concentrate the magnetic flux induced therein from an external source, e.g., current flowing through an electrical coil wrapped thereabout, and have little or no residual magnetism after the source is removed, e.g., current flow discontinued. Permanent magnets are similarly molded from such ferromagnetic materials as magnetic ferrites, samarium-cobalt alloys, iron-neodymium-boron alloys, and the like, and are subsequently permanently magnetized.
In making such moldings, the ferromagnetic particles have been mechanically mixed/blended with particles of the matrix-forming polymer, and the mixture compression molded in a suitable mold. Mechanical mixing/blending however, does not result in a uniform distribution of the ferromagnetic particles throughout the polymer matrix, or in insulation (e.g., electrical or protective) of each and every ferromagnetic particle from the next by the matrix-forming polymer as is required for many applications. Interparticle electrical insulation, for example, is necessary to reduce core losses in soft magnetic cores used in AC applications. In permanent magnets, interparticle protective insulation is desirable to protect the ferromagnetic particles from corrosion.
Magnets and soft magnetic cores have also been made by individually encapsulating each of the ferromagnetic particles within a soluble thermoplastic polymer shell formed by spray-coating the polymer onto the surface of each of the particles as they move in a fluidized stream through a coating zone in an appropriate coating machine. The thusly encapsulated particles are then compression molded to form the desired magnetizable article. For example, Ward et al. U.S. Pat. No. 5,211,896 discloses encapsulating soft magnetic iron particles in a shell of polyetherimide, polyamideimide or polyethersulfone which is spray coated thereon from a solution thereof. Similarly, U.S. Pat. No. Shain et al. 5,272,008, discloses iron-neodymium-boron permanently magnetizable particles having epoxy and polystyrene layers spray-coated thereon. By so individually encapsulating each and every ferromagnetic particle prior to molding, (1) a uniform distribution of the ferromagnetic particles throughout the polymer matrix is achieved, (2) clumping of the ferromagnetic particles and/or the matrix-forming polymer together is avoided, and (3) each of the particles is insulated from the next by the polymer.
According to Ward et al. U.S. Pat. Nos. 5,211,896 and Shain et al. 5,272,008, the thermoplastic polymer is dissolved in an appropriate solvent, and the particles spray-coated in a fluidized stream thereof. U.S. Pat. Nos., Smith-Johnson 3,992,558; Lindlof et al. 3,117,027; Reynolds 3,354,863; Wurster 2,648,609 and Wurster 3,253,944 inter alia are examples of coating apparatus suitable for this purpose. Preferably, Ward et al.'s and Shain et al.'s particles are coated using a Wurster-type fluidized-stream, spray-coating apparatus and method. Wurster-type equipment comprises a cylindrical outer vessel having a perforated floor through which a heated gas passes upwardly to heat and fluidize a batch of ferromagnetic particles therein. A concentric, open-ended, inner cylinder is suspended above the center of the perforated floor of the outer vessel. A spray nozzle centered beneath the inner cylinder sprays the thermoplastic solution (i.e., polymer dissolved in a solvent) upwardly into the inner cylinder as the fluidized ferromagnetic particles pass upwardly through the spray in the inner cylinder. The particles circulate upwardly through the center of the inner cylinder and downwardly between the inner and outer cylinders. The gas (e.g., air) that fluidizes the metal particles also serves to vaporize the solvent causing the dissolved thermoplastic to deposit onto the particles. After repeated passes through the coating zone in the inner cylinder, a sufficient thickness of polymer accumulates over the entire surface of each particle as to completely encapsulate such particle.
The choice of matrix-forming polymers available for individually encapsulating moldable, ferromagnetic particles as described above is limited in that useful polymers must satisfy several criteria. First, the polymer must be a thermoplastic in order to permit subsequent fusion and molding into finished articles. Second, the polymer must be durable enough to resist any chemically or thermally hostile environments in which the finished article is to be used. Third, the polymer must be sufficiently soluble in industrially acceptable solvents that it can be coatable in the manner described. Unfortunately, many polymers which satisfy the first and second requirements do not satisfy the third requirement, i.e., are not sufficiently soluble in acceptable solvents that they can be coated onto the ferromagnetic particles as described above.